This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Stable maintenance of gene expression or silencing is fundamental to hold proper cell identity over time. A paradigm for such function is provided by the transcriptional regulation of Hox genes mediated by proteins of the Polycomb (PcG) and Trithorax (TrxG) groups during animal development. Binding of PcG or TrxG complexes to defined cis-regulatory sequences, Pc and trx Response Elements (PREs and TREs) also termed Cellular Memory Modules (CMMs) in Drosophila, maintains over cell divisions chromatin structure in transcriptionally silent or active conformations. Convergent evidence indicates that distinct PcG and TrxG complexes exist, highlighting that their composition varies with regards to developmental time, axial position and target loci. This project aims to characterize the temporal dynamics of complex formation of a major PcG complex, the Polcyomb Repressive Complex 1 (PRC1) complex, purified from staged Drosophila embryos, and to analyze its function in stabilizing chromatin structure using developed and novel in vitro assays.